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594               Saturated and Superheated Steam.

the volume of i Ib. weight is 26*36 cubic ft, termed   *
volume : and the latter always = relative volume x •016*

Def. i. — The Saturation Point is attained iv*'

latent heat required for the steam has been  f " '

Def. 2. — The Boiling Point occurs when the tetf   *
water overcomes the surrounding pressure.

Def. 3. — Dry Saturated Stearn is that which k<* *
volume, pressure, and temperature, correspMt* * ;
complete formation.

Def. 4, — Wet Saturated Steam is in process </. ^
and is in contact with the water.

Def. 5. — Superheated Steam is that which h** '
perature raised above formation point.

Def. 6. — Specific Volume is the number of cttbi*

Ib. weight, and SPECIFIC DENSITY is the nuws?"
in a cubic ft.    (See pp. 766^^933.)

Dryness Fraction. — If the weight of water pat*11
given volume of wet steam be measured by suitable   ^
the proportionate wetness will be shewn  when  that    **-*
divided by the total weight of dry steam and water   4
while the proportionate dryness, or

TA            r     .•               weight of dry steam         (See Appen* f

Dryness fraction - -————-                ^

f  :                        thermometer,

Curves   of   Saturation   Points. — The   cont| «,!**"*'
temperature and pressure of  dry saturated   steam   I  * *
proved by experiment    From - 22° to 32°, Gay-Lus»»f    **«/**
apparatus in Fig. 604*   Both barometer tubes have vat *  *
the mercury, but B has a, little water on the surface of the*
whose vapour pressure reduces the height of the
i in. of mercury represents about •£ Ib. per sq. in., the          -w

therefore known.  Various freezing mixtures successivelf* * «^v,i
the blind end of tube 3, their temperature being

Fig. 605 was Regnaulfs apparatus for temperature*
to 122*.   As before, barometer A bus* a perfect
B*S vacuum is impaired by vapour,, rising from water              4